In magnetic-medium-based storage devices, data is typically stored on circular, concentric tracks on a magnetic disk surface. A read-write head retrieves and records data on the magnetic layer of a rotating disk as it flies on a cushion of air over the disk surface. When retrieving data, magnetic field variations are converted into an analog electrical signal, the analog signal is typically amplified, converted to a digital signal and interpreted. To guarantee the quality of the information saved on and read back from the disk, the read-write head should be very accurately positioned at the center of the track during both writing and reading. Frequently, a closed-loop servo system, driven by servo information embedded in a dedicated portion of every track of the recording surface, is used to accurately position the head and follow the track.
The servo information defines the position of the data tracks and thus should be written with great accuracy in order for a head servo system to operate properly. Typically, the servo information is written on each surface as a radially extending set of spokes or wedges. The portion of a servo wedge at a particular track location may contain a sync field, an index mark, a gray coded track number, and two or more fine-positioned offset bursts configured in an echelon across the track. Head positioning relative to a track center can be determined and corrected, if necessary, by reading and noting the respective amplitudes and timings of the offset bursts.
Traditionally, a machine called a servo writer is used to write the embedded servo information on the disk surface. Commonly, a servo writer uses a large, massive granite base to minimize the effects of vibration. The servo writer can also use precision fixtures to hold the target drive, a precision, laser-interferometer-based actuator arm positioning mechanism to place the arms radially with respect to the axis of rotation of the disks in the drive, and an external clock head to position the servo wedges in time. Present servo writers are typically large and expensive, and as the typical track density increases, the servo writing time also increases, which can create a bottleneck in the disk drive manufacturing process at the servo writer station.
Various attempts have been made to reduce usage of such servo writers. For example, some servo writing techniques have used a servo writer to generate high quality seed wedges. from which additional servo wedges can be generated by the disk drive itself using propagation self-servo write techniques. Other approaches have tried to eliminate the traditional servo writer altogether by pre-writing the disk with a low frequency reference pattern. Yet another approach has been to attempt an increase in throughput per servo writer by writing a spiral servo pattern on the disk, from which servo wedges can be generated by the disk drive itself.